1. Field of the Invention
The present invention relates to fracturing fluids of the type used to fracture subterranean formations and, more particularly, to a method for breaking a fracturing fluid through the use of a chemical agent incorporated within the viscosified fluid.
2. Description of the Prior Art
During the drilling of a well and the subsequent recovery of fluids from the well such as crude oil and natural gas, various materials are used to improve the efficiency of the well drilling operation, to increase the production of fluids from the formation and/or to plug or seal a non-producing well. For example, a subterranean formation is often subjected to a fracturing treatment to enhance the recovery of fluids such as crude oil or natural gas. During hydraulic fracturing, a sand laden fluid is injected into a well bore under pressure. Once the natural reservoir pressures are exceeded, the fracturing fluid initiates a fracture in the formation which generally continues to grow during pumping. The treatment design generally requires the fluid to reach maximum viscosity as it enters the fracture which affects the fracture length and width. This viscosity is normally obtained by the gellation of suitable polymers, such as a suitable polysaccharide. The gelled fluid can be accompanied by a propping agent which results in placement of the propping agent within the fracture thus produced. The proppant remains in the produced fracture to prevent the complete closure of the fracture and to form a conductive channel extending from the well bore into the formation being treated once the fracturing fluid is recovered.
The recovery of the fracturing fluid is accomplished by reducing the viscosity of the fluid to a low value such that it flows naturally from the formation under the influence of formation fluids. This viscosity reduction or conversion is referred to as "breaking" and can be accomplished by incorporating chemical agents, referred to as breakers, into the initial gel.
In addition to the importance of providing a breaking mechanism for the gelled fluid to facilitate recovery of the fluid and resume production, the timing of the break is also of great importance. Gels which break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Premature breaking can also result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture width in the fracture being created.
On the other hand, gelled fluids which break too slowly can cause slow recovery of the fracturing fluid from the produced fracture with attendant delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in at least partial closing and decreased efficiency of the fracturing operation.
For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured. Thus, to be satisfactory, gel viscosity should remain in the range from about 50 to 75% of the initial viscosity at temperature over the entire time, usually between one and eight hours, that is required to pump the fluid into the fracture.
Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within about 24 hours after completion of the fracturing treatment. A completely broken gel will be taken to mean one that can be flushed from the formation by the flowing formation fluids or that can be recovered by a swapping operation. In the laboratory setting, a completely broken, non-cross linked gel is one whose viscosity is about 10 centipoise or less as measured on a Model 35 FANN viscometer at 300 rpm or less than 100 centipoises by Brookfield viscometer.
The controlled degradation of water soluble polysaccharides, used as viscosifying agents in hydraulic fracturing treatments of oil and gas wells, is thus an important consideration. Historically, persulfate salts were added to the fracturing fluid to cause viscosity loss due to polymer degradation. Laboratory evaluations are routinely made before the treatment to find the persulfate concentration necessary to cause a reasonable viscosity decline. However, fracture conductivity studies have recently shown that the recommended persulfate concentrations are routinely inadequate to remove the residual fluid's impairment of the proppant pack.
The discrepancy in persulfate concentration is due, in part, to the filtration processes that occur during fracturing. During fluid and proppant injection, the solublized persulfate easily filters through the polymer filter cake formed at the fracture face. This persulfate laden filtrate is ultimately discharged into the formation reservoir where the oxidant harmlessly degrades. Consequently, the ratio of polymer to persulfate within the fracture is steadily increasing during injection.
One proposed method for controlling the activity of the breaker is described in U.S. Pat. No. 4,202,795 to Burnham et al. in which the breaker is introduced into the subterranean formation in the form of a prill or pellet formed by combining gel degrading substances with a hydratable gelling agent and forming the resulting mixture into the desired prill or pellet form. Upon exposure of the prills or pellets to an aqueous fluid, the gelling agent is said to hydrate and form a protective gel around each of the pellets, thereby preventing the release of the breaker into the aqueous fluid until the protective gel is broken by the gel-degrading substance. The cited reference claims that the breaker can be released to the aqueous fluid in a controlled manner by the described mechanism. It appears that a relatively large amount of the hydratable gelling agent is required to prepare the pellets and that the amount of hydratable gelling agent must be carefully controlled. In addition, the time period over which the pellets are released may vary substantially.
U.S. Pat. No. 4,506,734 to Nolte describes another method for delaying the release of a breaker by introducing a viscosity reducing chemical contained within hollow or porous, crushable beads into a hydraulic fracturing fluid. The viscosity reducing agent is said to be released upon the crushing of the beads which results from the closing of the fractures, caused by the fracturing fluid passing or leaking off into the formation or by removing the fluid by backflowing. However, stresses caused by the closing of the formation affect the percentage of beads being crushed so that a large percentage of beads may remain unbroken, particularly if the formation closes at a slow rate. Also, a large percentage of the beads may be crushed in one area of the formation being treated, whereas a secondary area of the formation may contain a substantially lower amount of beads to be crushed with resulting inconsistent performance.
A recent improvement to the above processes is described in U.S. Pat. No. 4,741,401 to Walles et al. in which an oxidant granule is encapsulated within a polymeric coating. The coating initially isolates the persulfate oxidant from the gelled fluid to minimize immediate viscosity declines while also allowing the granule to plate out in the filter cake. This places the oxidant in the filter cake to eventually degrade the polysaccharide both in the fluid and the filter cake. The persulfate is reportedly released by both permeation through the coating and by the crushing of the pellet by the proppant during fracture closure.
Although encapsulation offers certain advantages over the previously described prior art techniques, it also suffers from several disadvantages. For example, the coating thickness can be nonuniform, allowing a portion of the oxidant to be released immediately to cause initial fluid thinning. Secondly, at the high pressures encountered during fracturing, water permeates through the coating into the capsule interior. Heating from the reservoir and high concentrations of dissolved persulfate in the interior of the capsules cause nonproductive side reactions that continually consume oxidant. This loss of oxidant significantly affects the product's efficiency.
The present invention has as its object to provide a break mechanism for a gelled fracturing fluid which yields high initial viscosity with little change during pumping but which produces a rapid break in the gel after pumping is completed to allow immediate recovery of the fluid from the formation.
Another object of the invention is to provide a gel system for a well fracturing operation which will completely break within about 24 hours but which will maintain a satisfactory viscosity at operating temperatures for a time sufficient to complete the fracturing treatment.
Although the previously described methods describe delayed release of a breaker into a subterranean formation, the present invention has as its object to provide an alternative and improved method which provides a reliable, efficient and commercially viable breaking mechanism which is superior in performance to the previously described methods.